The present invention relates to a toner comprising toner particles each comprising a core provided with a conductive surface layer containing a doped electrically conductive polymer. A toner of this kind is known from the pre-published Japanese Patent Application 3-100561. This toner, which is made up of a number of individual toner particles, can be used, for example, in an electrophotographic imaging process. By varying, for example, the thickness of the conductive surface layer, the resistance of the toner can be adjusted between 1 and 1*1013 ohm*m. The toner resistance is practically independent of the ambient conditions, particularly the air humidity.
This toner differs particularly from toners coated with more conventional conductive surface layers, for example surface layers containing carbon, conductive metal oxides or conductive resins, and has the disadvantage that the resistance of this toner changes sharply when the toner is exposed to mechanical loads. After the toner is provided with a conductive surface layer, it is exposed to various mechanical loads. These include, for example, the loads accompanying the transport of the toner to an image-forming apparatus, particularly a printer. In the printer the toner again experiences a number of mechanical loads, such as being transported from an internal supply to a developing unit and the continuous supply of the toner to said developing unit. A change in the resistance has the effect that the development characteristic of the toner changes, and this has an adverse effect on the quality of a printed image. Problems of this kind can be obviated by continuously measuring the toner resistance in the developing unit and adapting the development settings to the measured value. However, this solution is expensive and increases the sensitivity of the image-forming apparatus to malfunctioning. A second possible solution is to adjust the development settings manually, either by a service engineer or by the user himself, for example if the imaging quality has perceptibly deteriorated. On the one hand, this solution has the significant disadvantage that the imaging quality is not constant, while on the other hand adjusting the development settings by a service engineer is expensive.
The object of the present invention is to provide a toner which has better resistance to mechanical loads. To this end a toner has been developed wherein an intermediate layer is provided between the core and the surface layer. It has surprisingly been found that the toner according to the present invention has a much better resistance to mechanical loads. As a result, the toner resistance changes much less sharply, so that the printing quality of an image-forming apparatus is much more stable over time. The reason why the toner according to the present invention has a much better resistance to mechanical loads is not completely clear. Possibly, the intermediate layer results in a foundation which is stable and homogeneous both physically and chemically so that the electrically conductive surface layer adheres more satisfactorily. Another possibility is that the intermediate layer provides a change in the morphology of the core surface so that the surface layer acquires a different structure which is more resistant to the typical mechanical loads. There is also the possibility of an interaction between the intermediate layer and the surface layer so that the latter becomes mechanically stronger. In one preferred embodiment, the intermediate layer contains a polymer. The advantage of a material of this kind is the low cost and minimal environmental load. In another preferred embodiment, the glass transition temperature of the polymer is 70xc2x0 C. or higher. As a result, the intermediate layer is practically unchangeable at the temperatures to which the toner is normally exposed in an image-forming apparatus, so that the good properties of the toner according to the present invention are maintained. In yet another preferred embodiment, the polymer has a glass transition temperature of 100xc2x0 C. or higher, whereby the toner has even more stable properties. Preferably, the polymer is transparent so that the toner can be colored by applying to the core a coloring agent, more particularly a dye or pigment. The advantage of this is that a strong color can be obtained in a simple manner. The polymer is preferably selected from the group consisting of polymethyl methacrylate, polyvinyl alcohol, polyvinyl pyrrolidone and copolymers of maleic acid and olefins. The said olefins can, for example, be aliphatic, alicyclic or aromatic and may carry one or more substituents. More preferably, the polymer is a copolymer of maleic acid and aliphatic olefins. A copolymer of this kind yields very stable toners.
The core of the toner preferably contains a fixable polymer, for example a thermoplastic polymer or a pressure-sensitive polymer. Common polymers are the styrenes, styrene copolymers such as the styrene acrylates, styrene-butadiene copolymers and styrene maleic acid copolymers, polyethylenes, polypropylenes, polyesters, polyurethanes, polyvinyl chlorides, epoxy resins and so on. These can be used as a single component or as a mixture. Preferably, the polymer has a weight-averaged molecular weight of between 200 and 100,000. This molecular weight can, for example, be adapted to the required mechanical properties of the image or to the intrinsic properties of the image-forming process.
The conductive surface layer preferably contains a doped electrically conductive polymer derived from one or more of the monomers selected from the group consisting of thiophen, aniline, pyrrole or derivatives thereof. A polymer of this kind contains a conjugate chain, so that charge carriers can readily shift. In this chain the charge carriers are created via a doping process, particularly a chemical or electrochemical process. Processes of this kind comprise an oxidation or reduction reaction, in which electrons are removed or added to the polymer chain. Preferably again, the surface layer contains polyethylene dioxythiophen. This conductive polymer has the advantage of being practically colorless, so that the conductive surface layer has no disturbing effect on the color of the toner. The surface layer may be a closed layer around a toner particle but it may also form an unclosed layer, particularly in the form of conductive paths. In a further embodiment, the core also contains a magnetisable material so that it can be used in an image-forming process making use of unary conductive magnetic toner.
The present invention will be further explained by reference to the following examples. All the reactions and experiments were carried out at room temperature.
Examples 1 to 6 describe how toner cores can be provided with an intermediate layer according to the present invention.
Examples 7 to 14 describe the preparation of a number of conductive toners according to the present invention.
Example 15 relates to an experiment concerning the resistance curve against mechanical loading of a number of toners according to the invention.